In view of the fact that a human spinal column is made up of a series of joined vertebrae, each of which has a specific curvature and a specific fixing direction, a surgical problem can often arise from the use of a vertebral fixation and retrieval system in fixing a vertebra under treatment. The Dimso Company, a French corporation, introduced the Diapason system which is disclosed in the U.S. Pat. No. 5,176,680 and is intended to provide a workable solution to the surgical problem arising from the use of the vertebral fixation and retrieval system. However, the Diapason system mentioned above is defective in design in that its fixation rod must be slightly curved in conformity with the curvature of a vertebra intended to be fixed. Such a surgical practice of curving the fixation rod of the Diapason system is in fact done in violation of the fixation principle of the spherical universal fixation device, thereby resulting in a poor fixation effect. In addition, the Dispason system is practically useless unless the bone screw and the fixation rod are so located that they are not far apart from each other.
The shortcomings of the Diapason system can be overcome by a universal fixation mechanism introduced by this inventor of the present application and disclosed in U.S. Pat. No. 5,352,226. However, the afore-mentioned universal fixation mechanism is limited in design in that it does not provide a workable solution to a minor surgical correction that must be done at such time when the fastening position of the bone screw is caused to deviate slightly.